1. Field of the Invention
The present invention relates to a system for and a method of controlling a two-shaft gas turbine.
2. Description of the Related Art
A two-shaft gas turbine is configured such that a high-pressure turbine driving a compressor and a low-pressure turbine driving a generator or a pump respectively have different shafts. This two-shaft gas turbine can rotate the compressor and the high-pressure turbine at high a speed even when a driven machine such as a pump and a screw compressor is rotated at a low speed. Therefore, the torque of the low-pressure turbine rotated at a low speed can be increased. For this reason, the two-shaft gas turbine is used to drive a machine such as a pump and a screw compressor. However, the two-shaft gas turbine also can be used such that the low-pressure turbine is used to drive the generator for electric power generation. When the two-shaft gas turbine is used without a speed reducer, there will be no loss resulting from the speed reducer and the compressor can be rotated at a high speed. The advantage is hence that higher efficiency can be achieved. Additionally, even when a speed reducer is used, a speed reduction ratio can be lower, which means the advantages in this case are a cost reduction and an improvement in efficiency. In the two-shaft gas turbine described above, the control system to achieve a high cycle compression ratio is disclosed JP-1988-212725-A, for example.
It is known in the art that when a gas turbine is operated at a high atmospheric temperature such as during summertime, intake air density lowers to lead to a reduction in the power output of the gas turbine. To deal with the reduction in the power output, there is a method in which, for example, droplets such as water are sprayed into the suction air of the compressor to increase the intake air density and, in addition, the intake air cooling has an effect of increasing the power output of the gas turbine. Further, when the droplets, of which the amount of spray is increased, are introduced into the compressor, the intermediate cooling has an effect of lessening the driving force for the compressor, which as a result improves the efficiency of the gas turbine. This is because the minute droplets carried into the compressor along with flowing air evaporate up to the saturation temperature of the stage while passing through the rows of rotor blades and the rows of stator vanes, and its latent heat of evaporation lowers the temperature of working fluid.
In a gas turbine (one-shaft type) provided with the output increasing mechanism by use of the water spray as described above, controlling the aperture of the inlet guide vane and the amount of spray of droplets at the intake portion of the compressor during rated operations is disclosed in JP-1999-72029-A, for example.